​Celtic have turned down Arsenal’s latest offer of £25m for left-back Kieran Tierney, with the Scottish side thought to be unhappy with the structure of the proposed deal. Having already seen a £15m bid knocked back, Unai Emery opted to return with an increased offer to try and tempt Celtic to part ways with one of their brightest talents. However, ​Sky Sports state that Celtic were less than impressed with ​Arsenal’s £25m proposal. They wasted little time in rejecting the offer, as they were not happy with Arsenal’s insistance of paying in add-ons. Instead, Celtic want more money upfront, and it is thought that they would accept an offer of £25m if they were to receive the amount immediately. This could be problematic for Emery, who is believed to be working with a transfer kitty of just £45m this summer. He has identified a number of targets, such as Wilfried Zaha and Malcom, although the Gunners may struggle to afford one of them, let alone several. Parting with over half of his budget to sign Tierney would leave Arsenal needing to sell a number of their first-team players if they are to have any chance of adding some new recruits this summer. The likes of Laurent Koscielny, Mohamed Elneny and Carl Jenkinson are all thought to be nearing the exit door at the Emirates Stadium, which would certainly raise funds to finance a deal for Tierney. ​Napoli remain keen on the left-back, but they are yet to make space in their squad as Mario Rui remains in transfer limbo. The 28-year-old is thought to be nearing a move to Benfica, but Napoli are reluctant to authorise a move for Tierney if there is a chance Rui’s move could collapse. However, the Partenopei are still determined to force through a move for Tierney, and they may be able to win the race for his signature if they can offer more money upfront.